Potassium currents in adult rat intracardiac neurones.

1. Properties of K+ currents were studied in isolated adult rat parasympathetic intracardiac neurones with the use of single-electrode voltage-clamp techniques. 2. A hyperpolarization-activated inward rectifier current was revealed when the membrane was clamped close to the resting level (-60 mV). The slowly developing inward relaxation had a mean amplitude of 450 pA at -150 mV, an activation threshold of -60 to -70 mV and a relaxation time constant of 41 ms at -120 mV. The current was reversibly blocked by Cs+ (1 mM) and became smaller with reduced [K+]o and [Na+]o, indicating that this inward rectifier current probably is a time- and voltage-dependent Na(+)-K+ current. 3. Step depolarizations from the holding potential of -80 mV evoked a transient (< 100 ms at -40 mV) outward K+ current (IA) which was blocked by 4-aminopyridine (4-AP, 1 mM). The time constants for IA inactivation were 20 ms at -50 mV and 16 ms at -20 mV. The steady-state activation and (removal of) inactivation curve showed a small overlap between -70 and -40 mV; the reversal potential of IA was close to EK. 4. Step hyperpolarizations from the depolarized potentials, i.e. -30 mV, revealed a slow inward relaxation associated with the deactivation of a time- and voltage-dependent current. The inward relaxation became faster at more hyperpolarized potentials and reversed at -85 and -53 mV in 4.7 and 15 mM [K+]o. This current was blocked by muscarine (20 microM) and Ba2+ (1 mM) but not affected by Cs+ (1 mM); this current may correspond to the M-current (IM). 5. Depolarization-activated outward K+ currents were evoked by holding the membrane close to the resting potential in the presence of tetrodotoxin (TTX, 3 microM), 4-AP (1 mM) and Ba2+ (1 mM). The amplitude of the outward relaxation and the tail current became smaller as the [K+]o was elevated. The outward tail current was reduced in a Ca(2+)-free solution and the residual current was eliminated by the addition of tetraethylammonium (TEA, 10 mM); the reversal potential was shifted in a direction predicted by the Nernst equation. These findings suggest the presence of delayed rectifier K+ current and Ca(2+)-activated K+ current. 6. Superfusion of TEA, Ba2+ and 4-AP, but not Cs+, induced rhythmic discharges in some of the otherwise quiescent intracardiac neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of muscarinic receptor mRNAs in rat heart and intrinsic cardiac ganglia by in situ hybridization.

Although the heart is considered a relatively pure source of m2 muscarinic receptors, the possible expression of other muscarinic receptor genes at discrete sites within the myocardium or by intrinsic cardiac ganglia had not been evaluated. Accordingly, the present study used in situ hybridization histochemistry with 35S-labeled oligonucleotide probes to address this issue. Initial experiments demonstrated that the localization of m2 mRNA was similar to that reported for muscarinic receptors labeled with the nonselective muscarinic antagonist quinuclidinyl benzilate; however, there were two important exceptions. The conducting system contained less message than expected, whereas the intrinsic cardiac ganglia contained more. The mismatch between muscarinic receptor and m2 mRNA densities in the conducting system could not be explained by the local expression of other muscarinic receptor genes, since m1, m3, and m4 mRNAs were not detected at this or any other site within the myocardium. However, the presence of a high density of prejunctional muscarinic receptors in the conducting system would be consistent with such a mismatch. Surprisingly, the intrinsic cardiac ganglia contained more than four times as much m2 mRNA as found in the atria. This level of message may be necessary for the production of prejunctional receptors on cholinergic nerve fibers within the heart and receptors localized to the ganglion cell bodies. The ganglia also contained smaller amounts of m1 and m4 mRNAs. These observations suggest that prejunctional muscarinic receptors could have a prominent role in regulating cholinergic neurotransmission in the conducting system and that multiple muscarinic receptors are present in the intrinsic cardiac ganglia.

Nicotinic and muscarinic synaptic transmission in canine intracardiac ganglion cells innervating the sinoatrial node.

Nicotinic and muscarinic mediated synaptic mechanisms were investigated in isolated, canine intracardiac ganglia taken from the right atrial fat pad. Using conventional intracellular microelectrode recording techniques on 216 neurons, fast and slow synaptic potentials were evoked by single or trains of stimulation of presynaptic fibers in interganglionic nerves. By varying the stimulus intensity, single or multiple fast excitatory postsynaptic potentials (f-EPSPs) were evoked, indicating the convergence of synaptic inputs on these cells. These f-EPSPs often reached the action potential threshold, were enhanced by the acetylcholinesterase inhibitor physostigmine and were blocked by the nicotinic antagonist hexamethonium. The f-EPSPs were accompanied by a decreased input resistance and had an extrapolated reversal potential of -7.1 mV, suggesting increased conductances to more than one cation. Repetitive presynaptic stimulation evoked slow excitatory postsynaptic potentials (s-EPSPs) in 41% of the cells while slow inhibitory postsynaptic potentials (s-IPSPs) or s-IPSPs followed by s-EPSPs were evoked in 19% of the cells. All slow potentials were abolished by atropine and low Ca2+/high Mg2+ solutions and enhanced by physostigmine. Hexamethonium and adrenergic receptor antagonists had no effects on s-EPSP and s-IPSP. The M1 receptor antagonist pirenzepine reversibly blocked the s-EPSP but not the s-IPSP. On the other hand, the M2 receptor blocker 4-diphenyl-acetoxy-N-methyl piperidine methiodide (4-DAMP) had no effects on the s-EPSP. These observations suggest that s-EPSPs and s-EPSPs are mediated by distinct muscarinic receptors. The amplitude of the s-EPSP and the depolarization evoked by the muscarinic agonist, bethanechol were accompanied by increased input resistance. These responses were decreased in amplitude by membrane hyperpolarization and either reversed polarity or declined to zero amplitude at about -80 mV, suggesting the inhibition of a potassium conductance.